In Table 4.4, Pearson correlations were run to assess the relationship between cortisol levels in saliva, serum and urine and meat pH, colour, thawing loss, cooking loss and tenderness in the 30 boars and 30 gilts aged 22 weeks. In terms of muscle pH, levels of cortisol in saliva after lairage (r = -0.38) and in serum (r = -0.35) were weakly negatively correlated (P <0.05) with muscle pH45. There was a lack of significant (P >0.05) correlation between cortisol
levels and measured muscle pH at 24 hours. With regard to meat colour, baseline saliva cortisol (r = -0.40) was negatively correlated (P <0.05) with the L* component of pork. Pearson’s correlation coefficient indicated that saliva cortisol after lairage was positively correlated with colour redness (a*) (r = 0.38, P <0.05). Baseline saliva cortisol (r <0.5) showed no significant (P >0.05) correlation with thawing and cooking losses and Warner- Bratzler Shear Force. Correlation analysis also demonstrated significant (P <0.05) negative correlations between saliva cortisol after transportation (r = -0.35), saliva cortisol after lairage (r = -0.44), serum cortisol (r = -0.40) and meat cooking loss.
85
Table 4. 4: Pearson’s correlation coefficients (r) among cortisol levels from saliva, serum and urine and measured pork quality variables
Variables BCSBT CSAT CSAL sCORT uCORT pH45 pHu L* a* b* C* Hb/a Tlos Cklos WBSF
BCSBT - -0.12 0.17 -0.004 -0.08 -0.07 -0.02 -0.40* 0.318 0.06 0.27 -0.17 -0.17 0.05 0.10 CSAT - -0.03 0.26 0.52* -0.18 0.13 0.25 -0.24 0.04 -0.19 0.22 -0.12 -0.35* 0.21 CSAL - 0.10 -0.09 -0.38* -0.05 -0.24 0.38* -0.01 0.31 -0.34 -0.10 -0.44* -0.31 sCORT - 0.09 -0.35* -0.12 0.08 0.09 -0.21 0.01 -0.29 -0.16 -0.40* 0.12 uCORT - -0.11 0.26 0.19 -0.31 0.07 -0.23 0.35 -0.20 -0.23 -0.30 pH45 - 0.21 0.16 -0.22 0.07 -0.15 0.25 0.24 0.25 0.12 pHu - 0.28 -0.20 0.13 -0.12 0.30 -0.07 0.03 0.28 L* - -0.46** 0.15 -0.32 0.55*** 0.15 -0.24 0.03 a* - 0.45** 0.96*** -0.38* -0.24 -0.15 0.40 b* - 0.69*** 0.64*** -0.05 -0.11 0.10 C* - 0.10 -0.21 -0.15 0.06 Hb/a - 0.19 0.01 0.05 Tlos - 0.27 -0.27 Cklos - 0.15 WBSF -
*** P < 0.001; ** P < 0.01; * P < 0.05; BCSBT = Baseline saliva cortisol before transportation; CSAT = Cortisol saliva after transportation; CSAL = Cortisol saliva after lairage; pH45 = pH at 45 minutes of slaughter; pHu = pH at 24 hours of slaughter; sCORT = Serum cortisol; uCORT
= Urine cortisol; sCK = Serum creatine kinase; L* = Lightness; a* = Redness; b* = Yellowness; C* = Saturation index; Hb/a = Hue-angle; Tlos =
86 4.4. Discussion
In this study, the absence of significant interaction between sex and lairage duration on muscle pH is in agreement with work by Hoffman and Laubscher (2011), Bureš and Bartoň (2012) and GarcÃa-Celdrán et al. (2012) who found no significance for pH measurements among female and male animals. In terms of muscle pHu values, they were within the
acceptable range (5.6 ≥ pHu ≤ 6. 00) according to Warner et al. (1997) and Mach et al.
(2008). According to Heap et al. (1998), Bertram et al. (2004), Hoffman et al. (2007), Lonergan (2008) and Peres et al. (2014), meat acidity/pH can have a profound effect on meat quality by directly affecting the functions of proteins (viz. solubility, binding and reflection).
The absence of significance on colour measurements in this study is in agreement with work by other researchers (Latorre et al., 2004; Jaturasitha et al., 2006; Beattie et al., 2007) who found significant differences for colour values among groups to be of little practical significance. The capacity of muscle proteins to hold fluids after external forces have been applied can influence the meat’s nutritional quality and its probability of being purchased (Jama et al., 2008). In this study, thawing loss and cooking loss were measured to determine the capacity of the sampled muscles to retain water after cooking. Despite them being not significant, the higher values for cooking loss in gilts than in boars were also reported by Latorre et al. (2004), Jaturasitha et al. (2006) and Đurkin et al. (2012) for Longissimus dorsi muscle. The absence of significant effect by lairage duration on cooking loss is in agreement with Toohey and Hopkins (2006) who found effect of lairage time on meat cooking loss to be insignificant. Lonergan (2008), Mach et al. (2008) and Đurkin et al. (2012) also reported that meat quality parameters including cooking losses and water-holding capacity can be predicted by pHu, whilst higher levels of intramuscular fat as associated with males are
87
Warner-Bratzler shear force (WBSF), a good predictor of tenderness in cooked meat (Liste et al., 2009) was used to determine tenderness of the sampled Longissimus dorsi muscles after cooking. In this study, neither sex nor lairage or their interaction was significant (P >0.05) on WBSF. These findings were in agreement with the reports by Liste et al. (2009) and Peres et al. (2014) who found no significance between boars and gilts, and no effect of lairage on tenderness. They attributed this to the insignificant variation both in muscle pHu and liquid
loss.
The demonstrated significant negative correlation between treatment saliva cortisol, serum cortisol and cooking loss may be explained by the status of the ionic content of muscle to hold natural water. According to Hedrick (1965), Kerry and Ledward (1999) and Tang et al. (2013), stress exposure prior to slaughter can alter muscle metabolism and subsequently, change the ultimate pH of the muscle (viz. 5.6 to 6.0) which can cause relatively great changes in meat colour and water-holding capacity. Hoffman and Laubscher (2011) echoed similar sentiments stating that high secretion of stress-related cortisol can alter muscle pH through its glycogenolysis and gluconeogenesis effects, coupled with protein degradation. The negative correlation between saliva cortisol after lairage, serum cortisol and muscle pH45
in our study may indicate that as cortisol levels increases due to stress, energy sources prior to slaughter would decrease, consequently high muscle pH45. This is in line with findings by
Okeudo and Moss (2005) which reported serum cortisol levels to be negatively related with pH45. However, this is contrary to a study by Škrlep et al. (2009) which reported positive
correlation between plasma cortisol and pH45, which could be reflecting cortisol response to
stress intensity prior to slaughter. Although there was no significant correlation between saliva cortisol levels and muscle pHu, it is interesting that boars had a greater cortisol
response to transportation which may indicate that, the higher L* value from boars is a result of greater cortisol response. Generally, the low lightness value (L* <40) observed between
88
sexes in this study may indicate that there was enough supply of muscle energy sources prior to slaughter hence, less pork lightness. This is supported by Škrlep et al. (2009) who noted that a positive correlation of cortisol levels with pHu indicate low levels of glycogen in the
muscle prior to slaughter hence, higher pHu (pHu >5.8). The observed negative significant
correlation between pork lightness (L*) and redness (a*) can be due to the high muscle ultimate pH (pHu >5.8) and status of the muscle protein. This is in agreement with Kannan et
al. (2003), Hoffman and Laubscher (2011) and Kadim et al. (2013) who reported that a low lightness (L* <40) colour component and high redness (a*) can be due to the increase in meat pHu and myoglobin content and thus, darker meat.
4.5. Conclusions
In the present study, sex had an effect on the meat lightness component (L*). Our study demonstrates significant sex by lairage duration interaction on meat cooking loss. In this study, lairage duration had no influence on any measured meat quality traits. Correlation analysis revealed a negative correlation between muscle pH45 and saliva cortisol after lairage
duration and serum cortisol. With regard to meat colour, a negative correlation between the L* component of pork and baseline saliva cortisol was observed. In contrast, a positive correlation was observed between meat redness (a*) component and saliva cortisol after lairage. The negative relations between saliva cortisol after transportation and lairage duration, serum cortisol and meat cooking loss could suggest that as cortisol secretion increases due to stress, the ability to hold fluids after external forces decreases. It was therefore concluded that transportation and lairage duration between sexes, saliva cortisol and serum cortisol had an effect on muscle pH45, colour lightness and cooking loss.
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